The present invention relates to a process for stripping light-hardened photoresist layers.
In the photoresist process, the steps of exposing and washing out the photoresist layer create an image stencil which shields underlying surface areas against etching or electroplating baths. When metal removal or deposition has ended, the photoresist stencil is customarily removed, in some cases by being peeling off mechanically, but more frequently by treatment with a sufficiently active chemical solution. This removal of the layer is referred to generally in the art as "stripping." The stripping solutions used for this purpose are typically solutions of strong bases, for example, an aqueous potassium hydroxide solution. Organic solvents are also frequently employed, usually with quaternary ammonium bases.
Different solutions which have been employed in removing various types of materials, including photoresist, are disclosed by the following documents:
(1) U.S. Pat. No. 4,089,704 describes the removal of a silicone rubber coating with a solution of tetramethylammonium hydroxide in methanol and isopropanol.
(2) U.S. Pat. No. 4,239,661 describes a cleaning solution for removing contamination, including fingerprints and positive photoresist residues, which comprises an aqueous solution of trialkylhydroxyalkylammonium hydroxide, a complexing agent and a nonionic wetting agent.
(3) U.S. Pat. No. 4,078,102 describes a stripping solution for negative- and positive-working photoresists that comprises an alkali metal, an alkaline earth metal or an ammonium hydroxide dissolved in an alcohol mixed with a carbonyl compound.
(4) U.S. Pat. No. 3,673,099 teaches that hardened resins, for example, of silicones or polyvinyl cinnamate, can be removed by means of a mixture of a strong base, for example a quaternary ammonium hydroxide, and N-methylpyrrolidone.
(5) U.S. Pat. No. 4,202,703 describes the stripping of a negative photoresist with a solution of tetramethylammonium hydroxide and a wetting agent in a lower alcohol, followed by dipping in trichloroethane.
In general, positive photoresist stencils are significantly easier to remove than negative resists. Since a positive resist comprises the unchanged, unexposed areas of the photoresist layer, it normally redissolves even in the coating solvent. By contrast, light-cured negative resists are substantially crosslinked as a result of exposure, so they are substantially insoluble in most solvents. Their stripping generally requires chemical agents that are very active. In addition, negative resists dissolve only incompletely, if at all, in stripper solutions, and instead form flocks of varying size which become deposited as a sediment. Depending on their size, these flocks can interfere considerably with the handling of the solution, particularly in automatic processing equipment.